The Islamic State terror group (ISIS) Tuesday issued a claim of responsibility for Sunday's attack on a Texas cartoon contest featuring images of the Muslim prophet Muhammad.

The claim was made in an audio message on the group's Al Bayan radio station, based in the Syria city of Raqqa, which ISIS has proclaimed to be the capital of its self-proclaimed caliphate. It is the first time ISIS has taken credit for an attack on U.S. soil, though it was not immediately clear whether the group's claim was an opportunistic co-opting of a so-called "lone wolf" attack as its own.

The message described the shooting suspects as "two soldiers of the caliphate" and added "We tell America that what is coming is more bitter and harder and you will see from the soldiers of the Caliphate what harms you."

The message also said the contest, which was being put on by a group known for controversial rhetoric about Islam, "was portraying negative images of the Prophet Muhammad."

House Homeland Security Committee Chairman Michael McCaul, R-Texas, told Fox News Tuesday that the attack was terrorism and at the very least inspired by ISIS.

McCaul also said in the days leading up to the attack, a joint FBI and Homeland Security bulletin was circulated and security for the event had been ramped up as a result, in Garland understanding that it was a target.

An investigation following the attack revealed a striking connection between at least one of the gunmen and a Twitter account based overseas, suggesting that ISIS operatives had knowledge of the attack beforehand and that the same fighters encouraged the shooters, a counterterrorism source told Fox News.

One British-based jihadi in Syria who does not tweet on a regular basis sent out a message within an hour of the attack, praising both men.

Another established ISIS Twitter account suggested he had been in contact with one of the gunmen just prior to the attack, using messages such as he tried to reach him but just missed him. The source said the social media appeared to show encouragement and mentoring.

The contest had been expected to draw outrage from the Muslim community. According to mainstream Islamic tradition, any physical depiction of Muhammad — even a respectful one — is considered blasphemous, and drawings similar to those featured at the Texas event have sparked violence around the world.

Authorities say the suspects, identified as Elton Simpson and Nadir Soofi, drove up to the building where the contest was being held in the Dallas suburb of Garland and opened fire. An unarmed school district security guard was wounded before a Garland police officer returned fire and killed both men.

Soofi had a long standing hatred of police and had studied overseas in Islamabad, Pakistan, according to a Facebook account that has since been disabled.

He once owned a pizza and hot wings restaurant in Phoenix called Cleopatra, but sold it years ago as it was struggling, the New York Times reports.

Public records showed that Soofi and Simpson were living in the same apartment complex in Phoenix, but it was not clear if they lived together, according to the newspaper.

Homeland Security Secretary Jeh Johnson said in a statement Monday that law enforcement authorities are investigating the men's motives and all circumstances surrounding the attack.

Court documents show that Simpson had first been noticed by the FBI in 2006 due to his ties to a a former U.S. Navy sailor who had been arrested in Phoenix and was ultimately convicted of terrorism-related charges. In 2010, Simpson was arrested one day before he was scheduled to fly to South Africa to undertake what he claimed were religious studies at a madrassa. Recordings played at Simpson's trial indicated that he was using his studies as an excuse to travel to Somalia to link up with militant fighters there.

Despite the more than 1,500 hours of recorded conversations, including Simpson's discussions about fighting nonbelievers for Allah, whom he referred to as "kuffars" the government prosecuted him on only one minor charge — lying to a federal agent. He faced three years of probation and $600 in fines and court fees.

There have been numerous attacks in Western countries believed related in some way to the group, which holds roughly a third of Iraq and Syria.

In October, Canada was hit by two terror attacks by so-called "lone wolves" believed to have been inspired by the Islamic State group. In Ottawa, a gunman shot and killed a soldier at Canada's National War Memorial and then stormed Parliament before being gunned down. Two days earlier, a man ran over two soldiers in a parking lot in Quebec, killing one and injuring the other before being shot to death by police.